1. Field of the Invention
The present invention relates to tuning assembly for tuning a dielectric resonator in a cavity defined by cavity walls. The invention also concerns a filter employing such a tuning assembly.
The dielectric resonator includes two resonator bodies, namely a stationary resonator body and a movable resonator body, each of the two resonator bodies being made of a low-loss, high dielectric constant material.
The tuning assembly comprises a support structure mounted in an opening in a mounting wall, constituting one of the cavity walls, for supporting the two resonator bodies within the cavity. The support structure includes two mutually slidable support elements, viz. a first support element, including a tubular sleeve portion, for supporting a first one of the two resonator bodies, and a second support element, including a shaft extending axially through the mounting wall opening and inside the tubular sleeve portion, for supporting a second one of the two resonator bodies. One of the support elements is displaceable from the outside by a tuning adjustment means to effect an adjustment movement of one resonator body in relation to the other resonator body, whereby a resonant frequency of the dielectric resonator in the cavity can be tuned.
2. Prior Art
Such a tuning assembly is previously known from the published international patent application WO 97/02617 (Allen Telecom). The known tuning assembly is disposed in a filter including a number of neighbouring cavities, each having a dielectric resonator and a plastic resonator support. The resonator support is mounted in one mounting wall only. Thus, unlike many similar support structures, it is not guided or supported in the opposite wall of the casing. A tuning assembly of the latter kind is disclosed in WO98/56062 (Allgon AB), the contents of which are incorporated herein by reference.
In one embodiment, shown in FIGS. 8 through 10 of the first-mentioned document WO 97/02617, the dielectric resonator includes two cylindrical resonator bodies in the form annular ring members, one stationary and one movable, both of them being made of a low-loss, high dielectric constant ceramic material. The first, stationary resonator body is mounted on a plastic support in the form of a cylindrical sleeve having a plurality of longitudinal recesses and openings so as to make the support somewhat flexible. At the inner end, to be located inside the cavity, the sleeve is cut out so as to form a number of spaced apart holding elements or arms diverging from a shoulder. When mounting the first, stationary resonator body onto the plastic support, it is pushed with its central axial hole onto the diverging arms.
When the first, stationary resonator body reaches a position where it abuts the shoulder of the plastic support, the arms will snap radially outwardly and engage with cantilevered stops onto the upper or inner surface of the cylindrical resonator body so as to hold the latter with a clamping force between the cantilevered stops and the shoulder. In this way, the first resonator body will be held substantially stationary by the plastic support.
The second resonator body, on the other hand, is secured to an adjustment shaft, which is mounted so as to extend through the mounting wall opening and axially inside the supporting cylindrical sleeve. The adjustment shaft is threaded at an axially outer portion thereof and is rotatable so as to perform a linear movement in relation to the plastic support and the first, stationary resonator body. The rotational movement can be accomplished manually, by means of a knurled outer head on the shaft, or automatically by a stepping motor. Thus, tuning can be achieved by such a movement of the adjustment shaft and an associated displacement of the second resonator body in relation to the first resonator body.
However, the plastic material of the support structure, which is necessarily flexible to enable the desired snap locking of the first resonator body, will inevitably make the mounting of the first resonator body somewhat resilient and not quite exact in a fixed position. Moreover, the adjustment shaft, which extends freely inside the support sleeve, is allowed to orient itself at a slight inclinational angle in relation to the support sleeve, whereby the second resonator body will be tilted in relation to the first resonator body.
Accordingly, the mounting of the first resonator body onto the support structure is not quite exact, and the tuning can only be achieved approximately, i.e. for a given rotational movement of the adjustment shaft, the mutual positions of the first and second resonator bodies can vary somewhat with an associated shift of the resonant frequency.